1. Field of the Invention
This invention relates to semiconductor devices, and particularly to a method for using an unclocked eFUSE (Electrically Programmable Fuse) circuit.
2. Description of Background
Electrically Programmable Fuses (eFUSEs) are widely used to implement memory redundancy functionality in dynamic random access memory (DRAM), static random access memory (SRAM), and embedded memory devices. Passing a sufficient current through an eFUSE structure typically programs the eFUSE, such that its resistance is significantly altered from its initially fabricated state. In order to determine whether a particular fuse has been programmed or not, a sense circuit may be used to detect one of two possible “states” of the fuse. More specifically, the sense circuit holds one of two latched values therein, which is driven by a comparison between an applied voltage (by the sense circuit) across the fuse and a reference voltage generated within the sense circuit. The reference voltage is designed to be between a fuse voltage corresponding to the programmed state and a fuse voltage corresponding to an unprogrammed state.
However, it is common for registers to be part of scan chains for test. In a particular military application, it is essential that no amount of hacking can cause the contents of internal registers to be scanned out. To accomplish this, a technique is required to use an unblown eFUSE to enable scan for test, but then to blow the eFUSE at the conclusion of test to disable scan. The eFUSE implementation must be extremely simple, and it must be unclocked so that it passes conservative military design reviews with consensus “100% certainty” that it will disable scan. Existing eFUSE macros are all clocked, and therefore they open the door that hacking could power up the chip without clocking the eFUSE macro and thereby leave scan enabled, accessing the internal registers. The exemplary embodiments of the present invention eliminate that exposure, and are simpler and smaller than a clocked eFUSE macro.
Considering the limitations of the above-mentioned methods, it is clear that there is a need for an unlocked eFUSE circuit.